The present invention relates generally to the processing of near-gamma titanium aluminides, and more particularly to a method for thermomechanically processing near-gamma titanium aluminides so as to break down the ingot coarse microstructure with either partial or full homogenization of the microstructure and to yield a largely equiaxed gamma microstructure.
The two phase near-gamma titanium aluminides are attractive candidates for applications requiring low density and high strength at elevated temperatures. One of the main drawbacks limiting their application is their low room temperature tensile ductility. It is known that one of the prime methods of improving ductility is to refine the gamma grain size of these materials.
FIG. 1 shows tensile data obtained in this investigation for a near-gamma titanium aluminide (Ti-48Al-2.5Nb-0.3Ta aim composition, in atomic percent), which illustrates the important trends. The data are for sheet samples, all of which contain a nominally equiaxed gamma grain structure, but some contain coarse grains (lower ductility data) and some contain finer grains (higher ductility values). To be precise the ductility values around 0.3 percent are for samples with a bimodal grain structure, but a peak grain size of 50 .mu.m, while those samples with ductilities around 0.8 percent had a uniform fine grain size of 15 .mu.m.
Two main techniques presently exist for primary consolidation of near-gamma titanium aluminides: powder metallurgy and ingot metallurgy processes. Powder metallurgy processes consist of some method of producing powder which is then consolidated by hot isostatic pressing (HIP'ing) followed by extrusion, etc. Such techniques are expensive, and even though such processes avoid the segregation of alloying elements and phases (i.e. alpha-two and gamma in the near-gamma titanium aluminides) they suffer from high levels of interstitials (C, O, H, N) which degrade properties, trapped inert gas (e.g., He), and problems with thermally induced porosity (TIP) during processing. Ingot metallurgy materials are fabricated via arc melting, HIP'ing (to seal casting porosity), isothermal forging or extrusion to break down the cast structure, and finish processing (e.g., rolling, superplastic forming, closed-die forging).
Ingot metallurgy processes are much less expensive and have the further advantage of much reduced interstitial levels.
The main drawback of ingot-metallurgy processing of near-gamma titanium aluminides is associated with the slow cooling after casting and the resultant segregation on a microscopic (as well as sometimes on a macroscopic) scale. Microsegregation is manifested by the development of dendritic regions, with an alpha-two/gamma lamellar two-phase structure, that are the initial solidification products, and interdendritic regions consisting solely of single phase gamma. During subsequent high temperature deformation (e.g., isothermal forging, rolling) and thermal processes, the cast structure is broken down to yield a refined structure. However, because of the difficulty of homogenization of the gamma phase even with deformation, broken down or wrought products exhibit the signature of the microsegregation developed in the ingot casting.
The signature observed by the present inventors consists of (1) fine equiaxed grains of gamma+alpha two that have evolved from the prior dendritic, lamellar two-phase region, and (2) regions of single-phase, coarse gamma grains. The coarse gamma grains are recrystallized from the prior interdendritic gamma, but in the absence of a second phase (e.g., alpha-two) have undergone grain growth at the required high processing temperatures. The bimodel grain structure is usually very undesirable.